Acute lung injury (ALI) and its severe form acute respiratory distress syndrome (ARDS) are devastating syndromes responsible for significant morbidity and mortality. The pathogenesis of ARDS is still poorly understood and therapeutic options remain limited. Hyperoxia-induced lung injury is an established model which mimics human ARDS and has been used extensively by investigators. Lung epithelial cell death is a key feature of ALI and is crucial in the pathogenesis of ALI /ARDS. Cell death is regulated by signaling molecules, which have been shown to congregate on lipid rafts. Caveolin-1 (cav-1) and flotillin1(flot1) have been identified as lipid raft marker proteins, which are highly expressed in various lung cells. However, the regulation and function of flot1 in respiratory biology has been poorly, if at all, studied. Based on our published and preliminary studies, we believe that cav-1 and flot1 are important effector molecules which play critical roles in the pathogenesis of ALI and hyperoxia-induced epithelial cell death. Our published work has demonstrated that cav-1 null mice are resistant to hyperoxia induced ALI. Since our previous submission, our newly published data further showed that cav-1 increases hyperoxia-induced apoptosis via suppressing survivin. In contrast to cav-1, our data showed that flot1 protects against hyperoxia induced cell death. Cav-1 and flot1 together regulate hyperoxia induced cell death via Fas pathways independent of FasL. Flot1 and cav-1 both interacted with Fas after hyperoxia, indicating that cav-1 and flot1 cross talk and mediate the death signaling. We hypothesize that cav-1, flot1 and their cross-talk modulate hyperoxia induced epithelial cell death and ALI via regulating Fas signaling pathways. We anticipate that our studies will lead to the identification of novel targets for the development of therapeutic approaches against acute lung injury. We will test our hypothesis in the following specific aims: Aim1I: To determine the functional role of cav-1 and the underlying mechanisms by which cav-1 mediates hyperoxia induced epithelial cell death. Aim 2: To determine the regulation and function of flot1 in hyperoxia induced lung epithelial cell death and lung injury. Aim 3: To determine the cross-talk between cav-1 and flot1 in hyperoxia induced lung epithelial cell death and lung injury.